1. Field of the Invention
The present invention relates to a boundary acoustic wave filter device used as, for example, a band-pass filter of a cellular phone, and, more particularly, to a longitudinally coupled resonator boundary acoustic wave filter device including a plurality of IDTs that are arranged in a boundary acoustic wave propagating direction.
2. Description of the Related Art
Surface acoustic wave filter devices are widely used as band-pass filters of cellular phones. For example, Japanese Unexamined Patent Application Publication No. 2006-87145 describes a surface acoustic wave filter device having an electrode structure shown in FIG. 7.
A surface acoustic wave filter device 1001 shown in FIG. 7 includes a piezoelectric substrate 1002. The electrode structure shown in FIG. 7 is arranged on the piezoelectric substrate 1002 so as to define first and second longitudinally coupled resonator surface acoustic wave filter portions 1011 and 1012. In the longitudinally coupled resonator surface acoustic wave filter portion 1011, a first IDT 1014 is arranged in the middle and second and third IDTs 1013 and 1015 are arranged on both sides of the first IDT 1014 in a direction in which a surface acoustic wave propagates. Reflectors 1016 and 1017 are arranged on both sides of the region in which the IDTs 1013 to 1015 are arranged in the direction in which surface acoustic wave propagates.
Similarly, in the second longitudinally coupled resonator surface acoustic wave filter portion 1012, a first IDT 1019 is arranged in the middle and second and third IDTs 1018 and 1020 are arranged on both sides of the first IDT 1019 in a direction in which a surface acoustic wave propagates. Reflectors 1021 and 1022 are arranged on both sides of the region in which the IDTs 1018 to 1020 are provided. An input terminal is connected to one end of the first IDT 1014 of the first longitudinally coupled resonator surface acoustic wave filter portion 1011. An end of the IDTs 1013 and 1015 are respectively connected to an end of the second and third IDTs 1018 and 1020 of the second longitudinally coupled resonator surface acoustic wave filter portion 1012 through signal lines. One end of the middle first IDT 1019 of the second longitudinally coupled resonator surface acoustic wave filter portion 1012 is connected to an output terminal. Ends opposite to the ends of the IDTs 1013 to 1015 and IDTs 1018 to 1020, to which the input terminal, output terminal and signal lines are connected, are all connected to a ground.
In the IDTs 1013 to 1015 and IDTs 1018 to 1020 of the surface acoustic wave filter device 1001, a narrow pitch electrode finger portion is provided in each IDT at a portion at which two IDTs are located adjacent to each other in the direction in which a surface acoustic wave propagates. For example, in the IDT 1013, a portion of the IDT 1013 including an electrode finger 1013a located at an end adjacent to the IDT 1014 and an electrode finger 1013b located next to the electrode finger 1013a has a relatively narrow pitch. In this manner, the narrow pitch electrode finger portion is provided. The electrode finger pitch of the narrow pitch electrode finger portion is less than the electrode finger pitch of the electrode finger portion of the IDT 1013 other than the narrow pitch electrode finger portion.
In the longitudinally coupled resonator surface acoustic wave filter device 1001, because the narrow pitch electrode finger portion is provided in each IDT at the portion at which two IDTs are located adjacent to each other, an insertion loss can be reduced in a pass band.
Recently, a boundary acoustic wave filter device that utilizes a boundary acoustic wave that propagates along a boundary between two media having different acoustic velocities has been used. An electrode structure of the boundary acoustic wave device may appropriately utilize the electrode structure of the surface acoustic wave filter device.
When the boundary acoustic wave filter device includes an electrode structure that is similar to that of the longitudinally coupled resonator surface acoustic wave filter device described in Japanese Unexamined Patent Application Publication No. 2006-87145, an insertion loss in a pass band may be reduced by providing the narrow pitch electrode finger portions. However, a large spurious response tends to occur in a frequency band that is higher than the pass band. Thus, the boundary acoustic wave filter device cannot be used for applications in which the occurrence of a spurious response in a band higher than the pass band is unfavorable.